Structure-property relations of cationically polymerized epoxy thermosets with different morphologies are examined. The morphology adjustment of amorphous epoxy based copolymers and partially crystalline polymer alloys is carried out with star-shaped poly(epsilon-caprolactone) (SPCL) bearing various numbers of hydroxyl end groups. These hydroxyl groups are known for their reactivity toward epoxides following the activated monomer (AM) mechanism. For this reason, four-armed SPCL was synthesized with four hydroxyl end groups (SPCL-tetraol) and, in addition, with successively esterified ones down to a SPCL with four ester end groups (SPCL-tetraester). SPCL species bearing fewer or no hydroxyl end groups segregate into needle-like nanodomains within the epoxy networks and, if the concentration is high enough, also into crystalline domains. The stronger phase separation of SPCL-tetraester within the epoxy network compared with SPCL-tetraol is due to a reduction of the AM mechanism. The mechanical properties resulting from different morphologies lead to a trade-off between higher storage moduli and T-g values in the case of the more phase separated (and partially crystalline) polymer alloys and higher strain at break in the case of the amorphous copolymers. Nevertheless, in both cases toughness is improved or at least kept on the same level as for the pure epoxy resin. (c) 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2188-2199